Spatial and temporal changes in host gene expression after virus infection

The study of virus-host interactions is complicated by the nature of the virus invasion of plant tissues. This is a progressive process. At the advancing infection front, newly infected cells show a short phase of virus replication and then a persistent phase when progeny virus remains in the cell without further replication. Hence infected tissues are collections of asynchronously infected cells. Recognizing this, we have used cell biological and molecular analysis of infected tissues to obtain snapshots of host responses across a dynamic infection front. Using this approach we have shown that there is a high degree of temporal and spatial regulation of host gene expression and host physiology in cells carrying out virus replication.

For example, consecutive sections of pea cotyledonary tissue, taken from PSbMV-infected plants show highly regulated induction of HSP70 and polyubiquitin RNAs associated with the virus infection front.

(Figure 1: shown using in situ hybridisation).

Another effect we see is the down regulation of expression close to the virus infection front.

(Figure 2)

Broadening the survey of host genes for additional virus plant interactions reveals a much more complex regulation occurring in advance of, at, and behind the advancing infection front.

From studies with PSbMV, PEBV, WClMV and BCTV in pea and CMV in cucurbits we can build a complex picture of gene regulation that involves local, remote (advance), and systemic induction of genes, a "shut-off" of much of host gene expression after a phase of virus replication (Figure 3). Some genes also remain unchanged in their expression (details can be found in the relevant publications).

(Figure 3)

To extend these studies, it will be necessary to establish more generic technologies for studying the response to virus infection in real time. For this, we have explored ways of generating synchronous virus infections in plant tissues and/or cell cultures on a time scale that more accurately represents a round of replication with a single cell (~5-8 hrs).  In this endeavour, we have been unsuccessful. As an alternative, we intend now to exploit the technology of laser capture microdissection (http://www.palm-microlaser.com/). RNA extracted from newly infected cells, dissected from locally infected leaf tissues, will be subjected to RNA profiling using microarrays for Arabidopsis thaliana and Nicotiana benthamiana.